Hydraulic pit prop



Aug. 23, 1960 F. G. REUTER HYDRAULIC PIT PROP Filed July 16, 1957 FIG 2 FIGI m 3 it irraaenfyl HYDRAULIC PIT PROP Franz Gottfried Renter, Lemforde, Hannover, Germany, assignor, by mesne assignments, to Mobay Chemical Company, Pittsburgh, Pa., a corporation of Delaware Filed July 16, 1957, Ser. No. 672,294 Claims priority, application Germany .Iuly 18, 19 56 5 Claims. (Cl. 121-48) This invention relates generally to hydraulic props and more particularly to props adapted for use in pits or caves of mines and the like.

Hydraulic pit props have been known which consist of a piston guided within a cylinder by piston rings, the piston rod end being the prop head. With such pit props, all seals, especially the piston rings, require close tolerances. Up or down movement becomes impossible if, due to slight denting, the piston gets jammed in the cylinder. It has been proposed to make pit props with resilient pressure bellows, but these could not be introduced into the mining industry for several reasons, mainly because of the relatively low strength of the resilient rubber bellows. Furthermore, the mechanical design of the bellow type pit prop is relatively complicated, so that there are no advantages compared to the hydraulic pit props.

It is, therefore, a primary object of the present invention to provide a hydraulic pit prop having a long service life even under the rough working conditions of mining operations. Another object of the present invention is to provide a hydraulic pit prop of sturdy construction. A further object of the invention is to provide a pit prop which can be handled easily. Still further objects will become apparent from the following description with reference to the accompanying diagrammatic drawing which illustrates a specific embodiment of the present invention.

In the drawing:

Figure l is a longitudinal sectional view of an embodiment of the invention before it has been extended; and

Figure 2 is a longitudinal sectional view of an embodiment of the present invention after it has been extended.

In accordance with this invention, generally speaking, the foregoing objects and others are accomplished by providing a hydraulic pit prop comprising tubes fitted inside each other in a telescope-like manner, said tubes enclosing a pressure chamber. More specifically, the invention provides a hydraulic pit prop comprising an outer tube and an inner tube having slidable contact with each other and enclosing a resilient hollow body, one end of said hollow body being attached to said outer tube and the other end of said hollow body being attached to said inner tube. This construction has the advantage that the tensional forces stored in the resilient hollow body after extension of the pit prop will, upon release of pressure, draw the latter together again without the need for mechanical parts, such as springs, being required to do this.

In order that the hollow body can withstand these high stresses, the invention provides for the hollow body to be made of seamless, highly resilient, polyurethane rubber. This seamless construction is possible by using suitable molding cores as disclosed and claimed in copending application Serial No. 647,155, filed March 19, 1957. A sufficiently good, highly resilient quality of the polyurethane rubber is achieved by the addition of activators, such as alkali salts of organic fatty acids, high Patented Aug. 23, 1960 molecular Weight alkyl arylsulfonates or alkylsulfonates, to the hydroxyl polyesters serving as the starting material for the production of the polyurethane rubber. The activators are generally used in quantities of 0.00-l-1% by weight, based on the weight of said hydroxyl polyester.

The invention furthermore contemplates a number of valuable design features, particularly with regard to the detachable connection of the resilient hollow body to the bottom of an outer telescope member and to the attachment of said resilient body to an end plate joined to the inner telescope member. Thanks to this construction as explained in detail below, the pit prop can easily be taken apart and, in the event of damage, the outer telescope housing or the inner hollow body can easily be replaced by spare parts.

Referring more particularly to the drawing, a commercial tube 1 closed by a welded-on bottom 2 serves as an outer housing for the pit prop. Into tube 1 is slipped'another commercial tube 3 with its bottom end ground to a taper. The top of tube 3 is closed by means of a plate 5, connected to it in a detachable manner with bolts 6. through which the hydraulic fluid enters the pit prop. Welded onto plate 5 is a tube 8 into which the nozzle of the supply nipple 7 protrudes. On the other end, tube 8 carries an end plate 10 with an orifice 11 which constitutes the connection to the supply nipple 7. Plate 10 is provided with dovetailed cutouts which engage the end part 12 of ahollowbody 14; The hollow body 14 consists of a highly resilient polyurethane rubber molded seam-less by using a-molding core and improved in its resiliency by the addition of activators, such as alkali salts, organic fatty acids, high molecular weight alkylarylsulfonates and alkylsulfonates, to the hydroxyl polyester reactant used in the production of the polyurethane rubber. This highly resilient hollow body is attached to the lower part of tube 1 near the bottom 2 by means of a pin 15 and is therefore connected to tube .1 as well as to tube 3. By supplying hydraulic fluid to the hollow body 14, tube 3 is driven out of tube 1 to the desired height as shown in Figure 2. No seal is necessary between the inside of tube 1 and the outside of tube 3, all that is required is that the tubes be guided inside each other. If the pit prop is to be taken out again, the hydraulic fluid is drained through the supply nipple 7 whereupon the tensional force of the hollow body 14 pulls tube 3 back into tube 1. If damage should occur to tube 1 or tube 3, the pit prop is easily disassembled by removing pin 15; tube 1 can then be straightened again or, after removal of the bolts 6, this can be done with tube 3 and the parts that are permanently attached to the hollow body 14. In addition to the dovetail connection, the hollow body is cemented onto plate 10.

Any suitable formulation may be used to produce the hollow polyurethane rubber bodies serving as pressure chambers in the hydraulic pit props of the invention. Suitable formulations and starting materials have been disclosed in the literature, such as US. Patents 2,729,618 and 2,764,565, the disclosure of which is incorporated herein by reference. In order to enhance the resiliency of the polyurethane rubber, a small amount of an activator of the type hereinabove disclosed is added to the hydroxyl polyester serving as the starting material prior to its reaction with the organic diisocyanate and the cross-linking agent.

The following working example shows how a polyurethane rubber of high resiliency can be prepared but it is to be understood that any other conventional formulation for making highly elastic polyurethane rubber can also be employed.

Into plate 5 is inserted a supply nipple '7' Example The polyurethane rubber is prepared from about 1,000 parts by weight of a linear polyester obtained by thermal esterification of adipic acid with ethylene glycol and having a molecular weight of about 2,000, an hydroxyl number of about 50 and an acid number of-about 1 heated at about 135 C. less than atmospheric pressure and preferably at a pressure approaching a vacuum for about one hour until substantially all the moisture has been removed. Then 0.1 parts sodium-salt of stearicacid has to be added to this polyester. About 180 parts by weight naphthylene-l,S-diisocyanate are next introduced into the mixture and reaction is brought about by allowing the mixture to set for about minutes while the pressure in the reaction vessel is less than atmospheric. About 14 parts 1,4-butanediol and 8 parts of trimethylol propane are added within about 1 minute, while stirring the mixture vigorously. The resulting mixture is poured into a suitable mold and maintained at about 110 C. for about 24 hours or until the product has cured to form a rubber-like polyurethane rubber of the desired size and shape for a hollow body.

Although certain representative embodiments and details have been shown for the purpose of illustrating this invention, it will be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit and scope of the invention except as set forth in the claims.

The physical characteristics of the polyurethane rubber are:

Elongation "about 300 to 700%.

What is claimed is:

1. A hydraulic pit prop comprising a rigid outer tube closed at one end and a rigid inner tube closed at one end in slidable relation one with the other and enclosing a resilient hollow body of seamless polyurethane rubber defining a pressure chamber, one end of said hollow body being detachably connected to the closed end of said inner tube through a supply nipple plate and the other end of said hollow body being attached to the closed end of said outer tube by means of a pin in a detachable manner.

2. A hydraulic pit prop as defined in claim 1 wherein said hollow body is seamless polyurethane rubber having 45 to 65 percent resiliency.

3. A hydraulic pit prop as defined in claim 1 wherein said supply nipple plate is connected to an end plate in which said resilient hollow body is anchored.

4. A hydraulic pit prop as defined in claim 3 wherein said end plate carries an orifice.

5. A hydraulic pit prop as defined in claim 3 wherein said supply nipple plate and said end plate are located at the ends of a tube.

UNITED STATES PATENTS 2,577,874 Carnahan Dec. 11, 1951 2,789,580 Woods Apr. 23, 1957 FOREIGN PATENTS 21,287 Great Britain Nov. 24, 1900, 3,742 France Dec. 11, 1904; (Addition to No. 328,360) 

